Theory for the short-time response of small Hg n clusters after excitation

The short-time behavior of small Hg _n clusters immediately after single or double ionization is studied. We calculate self-consistently the ground state electronic energyE of ionized Hg _n clusters. Upon ionization changes of the potential energy surface (PES) occur, which govern the atomic motion in the cluster. These changes depend on cluster size and charge and are determined by the interplay between the localization of the holes within an ionic core and the polarization energy of the neutral rest of the cluster. In the case of single ionization of the cluster the PES results mainly from hole delocalization. In contrast, in the case of double ionization the PES is governed almost only by strong environment polarization. We use our theory to explain the physical origin of the oscillations in the ionization cross-section of singly and doubly excited Hg _n clusters observed in recent pump-probe experiments.     

Metastable decay of Ar n + involving single monomer evaporation and the loss of peculiar numbers of monomers

Production and stability of Ar _n ^+* ions (withn up to 420) formed by electron impact ionization of a supersonic Ar cluster beam were investigated with a double focussing sector field mass spectrometer. The present study confirms previous magic number determinations up to the 4th icosahedral shell. A systematic study of metastable dissociations (monomer evaporation, magic number evaporation) for singly charged cluster ions as a function of cluster size, internal excitation energy and time elapsed since ion formation gives new insight into the ionization process and subsequent reactions of the ions formed. At a well-defined threshold energy ofca. 28 eV, the magic number loss mechanism occurs simultaneously with the well known single monomer evaporation process which proceeds at all energies. The new mechanism is the first known example of cluster ion metastability showing an exponential dependence on time, providing further evidence that the precursor parent cluster ion is produced in a specific energy state.     

Theoretical study of the stability of small triply charged carbon clusters Cn3+ (n = 3–12)

Using density functional theory, coupled cluster and multireference methods, dissociation energies and 3rd ionization potentials for, respectively, triply charged and neutral carbon clusters have been evaluated. The results show that the smaller C_n³⁺ clusters are metastable, i.e., they present a fragmentation channel with negative dissociation energy. The lowest dissociation channel always corresponds to evaporation of a singly charged carbon atom. Good agreement with available experimental data is found for most two-fragment channels. The third ionization potential of the corresponding neutral species decreases with cluster size.     

Fragmentation spectroscopy of heterogeneous clusters

Photoionisation experiments were performed with heterogeneous Ar-Xe-clusters produced by supersonic expansion of argon gas with small quantities of xenon added to it. A threshold-electron photoionisation (TEPICO) technique was used to obtain time of flight cluster mass spectra. These mass spectra show particularly strong intensities for Ar₁₂Xe⁺ and Ar₁₈Xe⁺ which are attributed to the extraordinary stabilities of these cluster ions. Maxima in the ionic size distribution around Ar₇Xe⁺ are related to a particular abundance ofneutral Ar₁₂Xe which is fragmented after ionization. These stabilities are explained in terms of geometries consisting of a central Xe atom or ion surrounded by shells of Ar atoms. Filled shells exhibit particular strong bonding because these exhibit the largest number of atom-atom bonds. This conclusion is supported by simple theoretical calculations. The ionization process is discussed in terms of two direct and one indirect ionization channels the latter one proceeding via an intermediate electronic excitation of the Ar component in the neutral cluster.     

Negative carbon cluster ion beams: New evidence for the special nature of C60

Cold carbon cluster negative ions are formed by supersonic expansion of a plasma created at the nozzle of a supersonic cluster beam source by an excimer laser pulse. The observed distribution of mass peaks for the C⁻_n ions for n > 40 demon- strates that the evidence previously given for the special stability of neutral C₆₀ and the existence of spheroidal carbon shells cannot be an artifact of the ionization conditions.     

Laser photoionization and spectroscopy of gas phase silver clusters

Silver clusters containing up to 40–50 atoms are produced by laser vaporization in a pulsed-nozzle molecular beam source and studied with laser photoionization mass spectroscopy. A variety of Nd:YAG pumped dye laser and UV excimer laser wavelengths are used to achieve ionization. Ionization dynamics are studied by varying the laser wavelength and fluence. Bracketing experiments under single-photon ionization conditions are used to estimate ionization potentials as a function of cluster size. An even-odd ionization potential alternation is observed with odd-numbered clusters (N=3, 5, 7 ...) having lower ionization potentials than adjacent even-numbered species. Shell closings at clusters containing 2, 8 20 and 40 electrons are observed consistent with a one-electron shell model picture of cluster electronic structure. Resonance-enhanced ionization produces a vibrationally resolved spectrum for the trimer, Ag₃, yielding an electronic state assignment and excited state vibrational frequencies. Fragmentation in dimer ionization via theE state at 249 nm establishes the dissociation energy of Ag ₂ ⁺ to be <2.1 eV.     

Ionization dynamics of van-der-Waals clusters

The ionization process of homogeneous and heterogeneous van-der-Waals clusters has been investigated using various ionization methods (electron bombardment, charge exchange, photoionization methods), and different analyzing techniques. Direct and indirect ionization processes can be distinguished in the experiments from the shape of the ionization curve which depends on the type of cluster. These features appear differently in homogeneous and heterogeneous systems: Homogeneous systems exhibit characteristic ionization efficiency curves where the direct ionization path appears as a sudden increase in the ionization efficiency while the indirect transition gives rise to a long drawn out tail extending to the true ionization threshold. In heterogeneous clusters the indirect ionization path proceeds via excited states of the component with the larger ionization potential and subsequent energy transfer to the other component. These transitions are shifted and broadened depending on the type of internal interaction. Conclusions are drawn concerning the geometry and the interaction potential inside the cluster. The resolution of the TEPICO (Threshold Electron Photo Ion Coincidence) experiments makes it possible to determine the kinetic energy release of the fragments. It is shown that the results are related to the stabilities of the cluster ions involved in the fragmentation chain. Results are presented for pure rare gas clusters (Ar _n , Kr _n , Xe _n ) and for mixed systems (Ar _n O_2m , Ar _n Xe, Kr _n Xe, (CH₄) _n Ne).     

The gas phase “matrix isolation” spectroscopy of CH3F

We report infrared photodissociation spectra for Ne, Ar, Kr, N₂ and CH₄ clusters which contain CH₃F chromophores. The CH₃Fv ₃ mode is excited with a line tunable CO₂ laser. Mass spectrometer detection of changes in the cluster beam intensity serve to partially distinguish the spectra of different size neutral clusters. Many spectra consist of rather broad, inhomogeneous profiles. For intermediate size Ar_nCH₃F clusters a sharp, narrow peak is observed in the spectrum. We assign this peak as due to a cluster in which a central CH₃F molecule is surrounded by at least a full shell of Ar atoms packed in a contracted icosohedral geometry. Because the Ar atoms in a gas phase cluster are unconstrained by an extended crystalline structure, the CH₃F dipole is more fully stabilized (and thus red-shifted) than in a solid matrix. The dependence of the observed spectrum on cluster size is discussed. For comparison, no comparable narrow spectral features are observed in Ar_nC₂H₄ cluster spectra. Clear evidence is also presented that the fragmentation of the neutral clusters upon electron impact ionization is fairly specific. Finally, we note that ionization of Ar_nCH₃F clusters sometimes produces Ar_nF⁺ ions. This is a fragmentation process which does not occur in free CH₃F.     

On the localization of electrons and holes in Hg n and rare-gas clusters

We have used a microscopic theory to study the size dependence of the degree of localization of the valence electrons and holes in neutral an ionized rare-gas-and Hg _n clusters. We discuss under which circumstances localization of the electrons and holes is favoured. We have calculated the ionization potential of Xe _n , Kr _n and small Hg _n clusters. Good agreement with experiments is obtained. We have also determined the dependence of the ionization potential on cluster structure.     

Electronic properties of isolated GaNAsM clusters: photoionization-, photodissociation- and photoluminescence quantum yields

Photoionization quantum yields of Ga_NAs_M clusters with N + M = 85 ± 5 atoms in the spectral range of ω = 3.5–6.4 eV have been investigated by measuring their total photoabsorption and photoionization cross sections. It is found that the photoionization quantum yields of these clusters are strongly increased by about two orders of magnitude against the values of bulk GaAs. Photodissociation of the cluster provides another efficient channel whereas photoluminescence plays a minor role for the observed cluster size distributions with N + M typically between 30 and 130 atoms.     

Comparison between positive and negative charging of helium droplets

Helium droplets of approximately 10⁴–10⁸ atoms have been produced in free jet expansions of liquid helium through a 5 μm nozzle into vacuum. The size distributions of the positively and negatively charged droplets were measured as a function of the electron emission current. A simple model has been developed to describe the charging process and formulas for production of singly and doubly charged droplets were derived. The ratio of the ionization cross section to the geometrical cross section and its dependence on N was obtained. In the experiment single negatively and positively charged droplets were observed. Only for sizes N larger than a certain threshold size N _th ≈ 2 × 10⁵ the positively charged droplets were found to be doubly ionized. These observations are in good agreement with the assumption, that the positively charge carriers are stable “snowballs” while the negative droplets contain an excess electron located in the inside within a metastably bound “bubble”. The threshold size N _th corresponds to a simple model in which for smaller droplets a positively charged cluster of about 50 atoms is ejected.     

Total ionization and electron attachment cross sections of CCl2F2 by electron impact

The absolute total ionization cross sections from threshold to 250 eV and dissociative attachment cross sections from zero to 10 eV have been measured for the CCl₂F₂ (dichloro-difluoro-methane) molecule by using a parallel plate condenser type ionization chamber. The maximum of the ionization cross-section curve was found to be at an energy of about 90 eV with a cross section of 1.44 × 10^?19 m². The attachment cross-section curve shows three peaks, the most intense being at zero electron energy with a cross-section value of 1.80 × 10^?20 m², and the other two at energies of 0.6 eV and 3.5 eV, respectively. The maximal relative error in cross-section values is 0.08, for electron energies larger than 0.4 eV.     

Mass spectroscopic study of some novel water clusters: (H2O) n + ;n>3

A near atmospheric pressure ion source with a β-emitter as electron source is used to inject ions into a supersonic water expansion. Cluster ions of the structure (H₂O)⁺ _n have been observed forn up to 8. Forn>3 these cluster ions cannot be obtained by ionization of water clusters in vacuum, but they can be grown in the cold environment of a supersonic beam. Extremely clean conditions are necessary for the observation of these cluster ions. The data can be explained by assuming that the local potential minimum calculated for the (H₂O) _n ⁺ ,n=2, potential hypersurface exists also forn>2. The model developed to explain these data is similar to that proposed for ionized rare gas clusters.     

Influence of fluorine substitution on the properties of CdO nanocluster : a DFT approach

Fluorine substituted cadmium oxide (Cd _n O _n?1F) cluster for n = 2–6 of linear, ring and three dimensional structures were studied using B3LYP exchange correlation function with LanL2DZ as basis set. Different isomers were optimized to obtain structural stability and various parameters such as dipole moment, HOMO–LUMO gap, ionization potential, electron affinity, stability factor, binding energy, vibrational studies and optical absorption were studied and reported. The stability of the cluster depends on the binding energy and vibrational intensity.     

On the structure and stability of Ar n and Ar n + clusters at finite temperature

For Ar_2–29 and Ar _2–29 ⁺ clusters at 20 K in the polarization model presented here the electrodynamical dipole-dipole many-body problem is solved selfconsistently with the Monte-Carlo method (MC) at 20 K, i.e. the instantaneous dipole-dipole interaction is solved to infinite perturbation order and in cluster expansion to the order of the cluster size. The long range many-body dipole-dipole interaction is coupled to exchange interaction by a modified effective dipole polarizability. This model will be compared to the dimer model and classical MC simulation of Ar _n . The resulting different magic numbers in the binding energies are discussed in this connection with different experimental techniques of cluster ionization. By the mean square cluster diameter a shape parameter is introduced and it is found that with this parameter structural form transition in cluster growth can be resolved, and surprisingly do not correlate with the magic numbers.     

Two-photon (A1B2 ← X1 A1) supersonic jet spectrum of aniline

The supersonic jet multiphoton ionization (2 photons to resonance, 4 to ionization) ¹A ← ¹X spectrum of aniline is reported in the 560–590 nm region. The two-photon ¹A ← ¹X spectrum is very similar to the analogous one-photon spectrum. In particular, ν₁₄, the Kekulé signature mode of two-photon ¹L_b substituted benzene spectra is not prominent, but two quanta of the amino inversion mode, ν_I, are. A dispartiy between theoretical calculations of the ¹A ← ¹X two-photon cross-section, and the thermal lensing cross-section reported by Rice and Anderson [J. phys. Chem. 90, 6793 (1986)] is noted. The theoretical considerations only account for about half the thermal lensing intensity.     

Dissociative ionization of 1,4-bis(2,5-phenyloxazolyl)benzene

Dissociative ionization of 1,4-bis(2,5-phenyloxazolyl)benzene (POPOP) molecule by electron impact in gaseous phase is studied. Potentials of appearance of some fragments of the molecule under study are determined from the experimentally measured dependences of ionization cross-section on the ionizing electron energy. A new ion with m/z = 144 [C₉H₆ON]⁺ is detected in the mass spectrum of the POPOP molecule, being complementary to the fragment with m/z = 220 [C₁₅H₁₀ON]⁺. The threshold of appearance of this ion is determined (E_ap = 9.51 eV) as well as the first ionization potential of the POPOP molecule and fragment ion appearance potentials.     

Observation and investigation of metastable decay series of (N2) n + cluster ions

N₂ cluster ions are produced by electron impact ionization of a supersonic N₂ cluster beam and analyzed with a double focussing sector field mass spectrometer. It is found that metastable N₂ cluster ions lose more than one N₂ molecule in the μs time regime and decay predominantly via sequential series (N₂) _n ⁺ *→(N₂) _n?1 ⁺ *→...→N ₂ ⁺ , evaporating a single monomer in each of these successive decay steps. The metastable decay rates determined in detail for cluster sizes 2≤n≤6 andn=20 lie between 1 and 10⁶s^?1. These rates(i) depend strongly on the time elapsed after ion formation and on the respective parent cluster ion size, and(ii) exhibit a quasiperiodic pattern in magnitude.     

Multiple ionization and Coulomb explosion of mercury clusters in femtosecond laser fields

We report on studies of multiple ionization and fragmentation of free Hg_n (n ≤ 80) clusters in the femtosecond time domain at wavelengths ranging from 255 nm to 800 nm. After excitation by single laser pulses of an intensity of 5 * 10¹¹ W/cm² we observe prompt formation of multiply charged Hg_n clusters. The Hg_n cluster size distribution observed up to n ≈ 80 shows in additon to singly charged also doubly and triply charged clusters with a surprisingly high amount of doubly charged clusters. The measured cluster size distribution is nearly independent of laser wavelengths. For higher laser intensities (2 * 10¹² W/cm²) we observe multiply charged mercury atoms up to Hg⁵⁺. At 10¹³ W/cm² molecules and clusters eventually disappear due to Coulomb explosion and complete Fragmentation. Only atomic ions, singly and multiply charged, with high kinetic energies are then observed.     

Structure and stability for (AlN) n + and (AlN) n + (n=1–15) clusters

Using density functional theory (DFT) method with 6-31G* basis set, we have carried out the optimizing calculation of geometry, vibrational frequency and thermodynamical stability for (AlN) _n ⁺ and (AlN) _n ⁺ (n=1–15) clusters. Moreover, their ionic potential (IP) and electron affinity (EA) were discussed. The results show that the electrical charge condition of the cluster has a relatively great impact on the structure of the cluster and with the increase of n, this kind of impact is reduced gradually. There are no Al-Al and N-N bonds in the stable structure of (AlN) _n ⁺ or (AlN) _n ^-, and the Al-N bond is the sole bond type. The magic number regularity of (AlN) _n ⁺ and (AlN) _n ^- is consistent with that for (AlN) _n , indicating that the structure with even n such as 2, 4, 6, ... is more stable. In addition, (AlN₁₀ has the maximal ionization power (9.14 eV) and the minimal electron affinity energy (0.19 eV), which manifests that (AlN)₁₀ is more stable than other clusters.